An internal combustion engine has been developed with improved charging efficiency obtained by means of a supercharger, such as a mechanical supercharger. When the mechanical supercharger is used, it is necessary to arrange it compactly in the space afforded or provided for the engine in the automobile. In a V-type internal combustion engine, because space on the side of the engine is especially narrow, the mechanical supercharger is located between two inclined cylinder rows. Such an arrangement is known from Japanese Utility Model Publication No. 61 (1986)-1624.
To obtain high volumetric efficiency, it was thought that an intercooler should be provided between a mechanical supercharger outlet and an intake manifold. In such an arrangement, however, intake tubes between the supercharger outlet and an intercooler inlet and between the intercooler outlet and the intake manifold have to turn and twist in order for them to be arranged compactly in the engine room. Such a tube layout is apt to increase the intake flow resistance of intake air. Increasing the intake flow resistance causes a decrease in engine output power and increases fuel consumption.
Recently, volumetric efficiency is improved by means of utilizing resonance effects of intake air. When resonance effects are used, it is necessary that resonance passages, which are arranged upstream of the intake collecting portion where discrete passages for cylinders are collected, be designed to have a desirable length in order to achieve optimum resonance effects in the range of predetermined engine speed.